Coal Stratigraphy and Reservoir Identification in the Powder River Basin Coal Bed

Methane Play

By

Jimmy E. Goolsby and Andrew K. Finley

Goolsby, Finley & Associates, LLC, Casper, WY

Coal stratigraphy in the Powder River Basin has been important to the mining industry for many years, but only as it pertained to strippable coal deposits. The USGS has been involved with coal mapping and stratigraphy periodically in the Powder River Basin for more than 100 years. Work by the USGS was accomplished by many different geologists, each doing detailed surface and near-surface (using strat or core holes) mapping on individual quadrangles. Others did reconnaissance geology over large areas.

The extent and relationships of coals in the subsurface of the basin were unknown until oil exploration in the basin became common. Coal stratigraphy throughout the PRB has become important to the oil and gas industry and various state and federal agencies during the recent coal bed methane boom. The pathways of movement for groundwater and bacteria (necessary for generation of biogenic methane) cannot be fully understood without a clear understanding of basin wide coal stratigraphy. Water quality data is of limited value if we know only the geographic location of production and not the precise stratigraphic location. We must understand the stratigraphic relationship of producing coals to other coals and porous sandstones in order to make informed and intelligent decisions regarding gas and water production and their potential impacts or lack thereof.

Goolsby, Finley & Associates, LLC evaluation of coals based on more than 17,000 data points covering more than 7 million acres in the Powder River Basin has led us to the conclusion that most of the Late Paleocene coals are lithostratigraphically continuous. They are the result of continuous organic accumulation in a migrating depocenter, which co-existed with a complex fluvial system. This presentation will review regional stratigraphy, illustrate selected stratigraphic relationships and discuss their potential effects on gas and water production.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming